Our lab has studied the development of NK cells from stem cells and the acquisition of ki k:r immunoglobulin-like receptors (KIR) using in vitro models of differentiation. However, little is known about how and when NK cell receptor fate is determined in the setting of developing NK cells //;vivo. This question will be addressed by studies of NK cell reconstitution in patients undergoing unrelated marrow transplant with the goal of improving transplant outcome:;. Critical to the interpretation of these studies are controls for the genetic components that determine KIR expression. Allogeneic transplant is optimal for this study because each recipient is expected :o developdonor derived NK cells, that can be compared with their paired normal donor. The National Marrow Donor Progiam Sample Reposito/y will be invaluable tor the success of these studies. This resource, along with our novel system to quantitatively measure the expression of individual KIR gene products,will allow us to understand KIR expression and how it impacts on transplant outcomes. We have shown that KIR expression and NK cell function is markedly diminished after unrelated donor transplant in many recipients. The central hypothesis to be tested in this Project is that donor-de'ived NK cell function reconstituting after unrelated donor transplantation determines ^Jjnical iHiicpjnes. Function is determined by KIR acquisition mismatched for recipient MHC class I ligands on residual tumor cells cr APC to prevent and GVHD. Our current progress suggests that several signals influence KIR expression including 1L-3 and Il.-l'i, possibly through STATS pathways. We also address whether 1) KIR acquisition signals are mediated in an NK cell precursor, 2) IL-3 [unreadable] IL-15 induces a transcription factor in NK precursors which serves as a molecularswitch for KIR acquisition, 3) encountering class I MHC during development plays a role in the final KIR repertoire and 4) CD56*bngi" NK cells represent a distinct NK cell lineage after transplant, partially explaining the diminishedexpression of KIR aid other functional changes of biologic importance. The investigations are divided into the following three related specific aims: In Specific Aim 1 NK cell receptor reconstitution after unrelated donor transplant will be studied and the influence of NK cell receptor repertoires on clinical outcomes will be determined. In Specific Ann 2 we will assess the developmentof NK cells and their precursors /'//vivo. In Specific Aim 3 mechanisms of KIR acquisition in developirg NK cells will be studied.